pwm_out: limit actuator control callbacks to primary groups

- add backup schedule at minimum PWM rate
 - fixes #15043

src/drivers/pwm_out/PWMOut.hpp

@@ -161,6 +161,8 @@ private:
 
 	Mode		_mode{MODE_NONE};
 
+	uint32_t	_backup_schedule_interval_us{1_s};
+
 	unsigned	_pwm_default_rate{50};
 	unsigned	_pwm_alt_rate{50};
 	uint32_t	_pwm_alt_rate_channels{0};